Some private businesses may elect to prohibit you, though this is not very common. All prohibited locations, whether required by law, or by their own choice, are required to post a standard sign. The standard sign looks like this:

Private residences (home, apartment, etc.) are not required to post the sign. If you are going to be a guest in someone else's home,you may want to ask their permission first.

Parking Lot Exception

If a location is posted as no-carry, you may still retain your firearm in your vehicle. Before going in the location, you need to unload and store your firearm in your vehicle, and may do so in your car in the parking lot, or by getting out of your car and immediately storing your firearm in your trunk.

Walking Path Exception

If a walking or bicycling path passes through a park (has at least two entry/exit points at edge of park), then you may carry while on the walking / bicycling path. You are not suppose to leave the path with your firearm.

Federal Prohibited Locations

The rules for federal prohibited locations are different than the rules for Illinois prohibited locations. Most importantly:

Federal prohibited locations are not required to post a standardized sign; the signs vary in design

Federal prohibited locations often do not have a parking lot exception